Heart disease is the leading cause of a death in the United States and presents a major health risk for millions of people across the world. The cost to diagnose, treat and support patients suffering from various types of heart disease, including hypertrophic cardiomyopathy, myocardial infarction, and heart failure, is very high and puts a serious burden on the healthcare system.
Hypertrophic cardiomyopathy (HCM) is of particular concern because it is a significant cause of sudden unexpected cardiac death and is frequently asymptomatic prior to onset of cardiac arrest. HCM is characterized by a thickening of myocardial cells, which can affect ventricular function and give rise to cardiac arrhythmias. HCM is a primary genetic disease of the heart, resulting from a dominant mutation in a number of sarcomeric genes, of which 8 have been extensively studied (Teekakirikul et al., “Hypertrophic cardiomyopathy: translating cellular cross talk into therapeutics,” J. Cell. Biol., 2012, 199(3), 417-421). Of these dominant mutations in HCM, roughly 40% are missense mutations in the β-myosin gene, MYH7. Genetic and pharmacologic studies have shown reductions in expression of this dominant allele, even to a small degree, can have robust phenotypic effects (Jiang et al., “Allele-specific silencing of mutant MYH6 transcripts in mice suppresses hypertrophic cardiomyopathy,” Science, 2013, 342(6154), 111-114).
Myosin is the major contractile protein of cardiac and skeletal muscle cells. Cardiac muscle contraction depends on the expression and relative ratios of two myosin heavy chain (MHC) proteins, α-MHC (MYH6) and β-MHC (MYH7). In rodents, α-MHC, a fast-twitching MHC, is the predominant myosin isoform in the adult heart, whereas β-MHC, a slow-twitching MHC, is predominantly expressed in the developing heart and is downregulated after birth (Morkin, E., “Control of cardiac myosin heavy chain gene expression,” Microsc. Res. Tech., 2000, 50, 522-531). In contrast, in human heart, the β-MHC isoform is heavily expressed and the α-MHC isoform accounts for less than 8% of total ventricular MHC (Miyata et al., “Myosin heavy chain isoform expression in the failing and nonfailing human heart,” Circ. Res., 2000, 86(4):386-90). Irrespective of the differences in the expression of alpha- and beta-myosins in various species, studies indicate that expression of beta-myosin is upregulated in cardiac disorders in these species including humans, rats, and rabbits. For example, in failing adult mouse hearts, a shift from the normally predominant alpha-MHC toward beta-MHC is often observed (Harada et al., Circulation, 1999, 100, 2093-2099). Similarly, in rats, congestive heart failure was associated with increased expression of beta-myosin and decreased expression of alpha-MHC. Consistent with rodent studies, beta-myosin expression was upregulated and alpha-myosin expression was significantly down-regulated in failing human hearts (Miyata et al., “Myosin heavy chain isoform expression in the failing and nonfailing human heart”, Circ Res. 2000 Mar. 3; 86(4):386-90). These studies show that although the expression of alpha- and beta-myosins is species-dependent, downregulating the expression of beta-myosin is likely to play a cardioprotective role in various species. For example, blunting the increase in β-MHC expression and increasing α-MHC expression has shown to be cardioprotective in rabbits (James et al., “Forced expression of alpha-myosin heavy chain in the rabbit ventricle results in cardioprotection under cardiomyopathic conditions,” Circulation, 2005, 111(18), 2339-2346).
The genes encoding α-MHC (MYH6), β-MHC (MYH7), and a related myosin, MYH7B, also encode a family of intronic miRNAs, miR-208a, miR-208b, and miR-499, respectively. These three miRs share sequence homology and are called the “MyomiRs” (van Rooji et al., “A family of microRNAs encoded by myosin genes governs myosin expression and muscle performance,” Dev. Cell, 2009, 17, 662-673). MyomiRs have been shown to control pathological cardiac remodeling, muscle myosin content, myofiber identity, and muscle performance (Liu and Olson, “MicroRNA regulatory networks in cardiovascular development,” Dev. Cell, 2010, 510-525).
Although the role of α-MHC, β-MHC, and myomiRs (miR-208a, miR-208b, and miR-499) in cardiac and muscle development have been extensively studied, the role of the third myosin, MYH7B, is largely unknown. The MYH7B gene is expressed in skeletal muscle, heart, and in a subset of cells in the brain where it regulates synapse structure and function in the brain (Yeung et al., “Myh7b/miR-499 gene expression is transcriptionally regulated by MRFs and Eos,” Nucleic Acids Res., 2012, 40(15):7303-18). A recent report suggests that MYH7b protein is detected in a minor fibre population in extraocular muscles, corresponding to slow-tonic fibres, and in bag fibres of muscle spindles (Rossi et al., “Two novel/ancient myosins in mammalian skeletal muscles: MYH14/7b and MYH15 are expressed in extraocular muscles and muscle spindles”, J Physiol., Jan. 15, 2010; 588(Pt 2):353-64). However, in human heart, MYH7B gene is considered to undergo non-productive splicing, and result in a RNA that may not encode a functional MYH7B protein (Bell et al., “Uncoupling of expression of an intronic microRNA and its myosin host gene by exon skipping,” Mol. Cell. Biol., 30, 1937-1945). Interestingly, the non-productive splicing of MYH7B mRNA in heart cells was not associated with altered expression of intronic miR-499. Thus, until the present invention, MYH7B was considered to be a non-functioning carrier of miR-499 in cardiac cells (Gerald Dorn, II, “MicroRNAs: redefining mechanisms in cardiac disease,” J. Cardiovasc. Pharmacol., 2010, 56(6), 589-595). Moreover, unlike miR-499, MYH7B is considered not to play an important role in cardiac and skeletal muscle biology.